Indirectly heated cathode



Aug. 10, 1937. P, w STUTSMAN 2,089,817

INDIRECTLY HEATED GATHODE Filed Jan. 15, 1935 [NVE/VTOR PAUL 1M5rursMA/v A TTORNE Y Patented Aug. 10, 1937 UNITED STATES PATENT OFFICEmnnmo'ru HEATED oa'rnonr.

Application January 15, 1935, Serial No. 1,931

; 8 Claims. (Cl. 250-475) This invention relates to indirectly-heatedcathodes for electrical space discharge tubesfand more particularly tosuch cathodes which are heated by a heating filament.

In cathodes of the above type, it is often desirable to energize theheating filament by alternating current. Despite the fact that theelectron-emitting member of the cathode is usually electricallyseparated from the heating filament,

it has been difficult to eliminate disturbing effects due to thealternating character of the current. These disturbing effects have beenconsidered as being due to varying magnetic orelectrostatic fields or tovariable leakage currents between the electron-emitting member of thecathode and the heating filament. These leakage currents may be due toelectron emission between the electron-emitting member of the cathodeand the heating filament, or to direct leakage paths between these twomembers. These various disturbances have manifested themselves as anaudible hum in the output of systems in which such space discharge tubeswere used.

An object of my invention is to produce a cathode of the type designatedwhich is simple and inexpensive to manufacture, in which humproducingdisturbing effects are substantially eliminated by the elimination ofsubstantially all stray electrostatic and magnetic fields.

0 Another object of my invention is to decrease said hum-producingdisturbances by substantially eliminating variable leakage currentsbetween the electron-emitting member of the cathode and the heatingfilament.

The foregoing and other objects of my invention will .be best understoodfrom the following description of exemplifications thereof, referencebeing had to the accompanying diagrammatic drawing, wherein:

Fig. 1 is a representation if apparatus for producing heating membersfor my novel cathode;

Fig. 2 is a perspective view of a section of my novel heating elementshowing the various elements thereof broken away;

Fig. 3 is a side view of one form of my novel cathode mounted on thestem of an electrical space discharge tube;

Fig. 4 is a cross-section taken along line 4-4 of Fig. 3; and

Figs. 5 and 6 are diagrammatic representations of two different methodsof connecting the heating filament of my novel heater member.

My novel cathode utilizes as one of its novel elements a heating membermade-from a "twin" 55 wire heater, such as illustrated in Fig. 2, whichmay be formed by a continuous manufacturing process, such as thatillustrated in Fig. 1. The twin wire heater consists of two filaments Iand 2 placed side by side and embedded in an insulating coating 3. Thefilaments I and 2 are first separately coated with individual insulatingcoatings 4 and 5 in any suitable manner, such as by the well-knowndipping and baking process. The insulating coating which I prefer to useconsists of aluminum oxide. I prefer to coat the filaments I and 2 witha thickness of coating 4 and 5 which is substantially equal to thediameter of the filament. For example, if a filament of 2.3 mils isused, the coated diameter thereof would be not less than 7 mils, andpreferably slightly in excess thereof.

The filaments I and 2 carrying their respective coatings 4 and 5 may bemounted on spools 8 and I, such as that shown in Fig. 1, and thesespools are then placed in a coating apparatus, such as that illustrateddiagrammatically in Fig. 1. The coated filaments I and 2 are firstpassed over rollers 8 and 9 placed such as to bring the coated filamentsI and 2 into contact with each other and maintain them in contact. Thetwo separately coated filaments I and 2 are then passed through aninsulating coating bath I0, in which insulation, preferably in paste orliquid form, is applied by means of a roller II. The two coated wires Iand 2 with an additional outer coating of insulation are then passedthrough a furnace of some suitable kind, such as for example an electricoven I2. The usual coating apparatus of the type illustrated consists ofa series of such coating baths I0 and ovens I2. After the coated pair offilaments I and 2 has been passed through the successive coating bathsand ovens, the final insulating coating 3, as shown in Fig. 2, is formedthereon. As the finally coated wire comes from the apparatus, it passesover a guiding roll I3 and is wound up on a spool I4. If there are notsufiicient coating baths in a coating apparatus to give to the finalheater element the requisite thickness of coating, it may be necessaryto pass the heater element through the coating apparatus more than once.For example, in coating apparatus having three coating baths, I havefound that it is desirable to pass the heater element through theapparatus three times.

The heater element, as it is finally assembled on the spool I4, consistsof the two filaments I and 2 embedded in the insulating coating 3, asshown in Fig. 2". The outer surface of the heater element is ellipticalin shape, as illustrated ing coating to any one of the heater wires isless than this amount, a decided increase in hum is produced.

In order to construct the cathode with the heater element as produced bythe above process,

the coated heater element is cut off to the desired length. Theinsulation is cleaned oil the filaments at short distance at each end ofthe length cut off. The length is then folded over at its midpoint so asto form two halves I5 and I6 of the heater element which are parallel toeach other, as shown in Fig. and 4. Since the bend shown at the top ofFig. 3 is rather sharp, the insulation has a tendency to break off fromthe filaments at this point. However, the insulation on the remainingportion of the heater element spaces the two filaments from each otherand from the rest of the cathode structure. I prefer to bend the heaterelement so that the short axes of the cross-section of the halves I5 andI6 are in line with each other. The heater element bent in this manneris then slippedinside the cathode sleeve I I which may be provided onthe exterior with some suitable electron-- emissive coating IS. Theinterior diameter of the sleeve I1 is made substantially the same as thetotal distance between the outer surfaces of the two halves I5 and I6,and preferably slightly larger than said distance to facilitate easyassembly. Due to the manner in which the heating element is bent, thesleeve I'I touches the heater element only at points I9 and 20 which lieat the outer extremity of the short axes of the cross-sections of thehalves I5 and 5.. These points, as will be seen, are intermediate thefilaments I and 2.

The cathode assembly in the manner described is preferably connected inthe tube so that heating current flows through the filaments I and 2 inopposite directions to each other. For ex- 0 ample, one connection isillustrated in Figs. 3

and 5. In these figures, filaments I and 2 of the half I5 are welded tolead-in wires 2| and 22 sealed in stem 23 which may form part of anelectrical space discharge tube. The two filaments I and 2 of the halfI6 are welded together to a dummy wire 23 also sealed in the stem 23.The stem 23 is also provided with the usual additional supportingstandards for supporting the other electrodes of the tube. The twolead-in wires 2i and 22 are connected externally to some suitable sourceof heating current, such as, for example, a heating transformer 25. Thedummy wire 24 is not provided with any external connection. The heatingtransformer 25 mayhave its secondary 26 provided with a center tap 2'Iwhich is connected to a lead-in wire 28 which in turn is connected tothe cathode sleeve II by means of a conducting strip 29. The secondary30 of the transformer 25 is adapted to be connected to a suitable sourceof alternating current. To the lead-in wire 28- are also made the othercathode connections of any circuit, of which the space discharge tubemay be utilized. The resultant circuit for the heating filament,

illustrated in Fig. 3,'is shown in Fig. 5. 'The heating current fiowsfrom one of the lead-in wires, for example 2I, through the filament I ofboth halves I5 and I6, to the .point where both the filaments I and 2are connected to the dummy wire 24. Due to the fact that both of thefilaments I and 2 are welded to this dummy wire, an electricalconnection is made at this point so that the current then flows backthrough the filament 2 to the other lead-in wire 22. In this way the twofilaments I and 2 are connected in series with each other. Instead ofsuch a series connection, the two filaments I and 2 may be connected inparallel with each other, for example as shown in Fig. 6.

In each case I connect the filaments I and 2 so that heating currentflows through the filaments l and 2 in opposite directions at the sametime. Due to the fact that the filaments I and 2 are spaced close toeach other, there is very little resultant magnetic field which extendsout beyond the immediate vicinity of the two filaments. Also anypotential which exists at any point on filaments I and 2 has an equaland opposite potential at an adjacent point on the opposite filament,and thus, due to the fact that these filaments are placed very closetogether, there is very little resultant electrostatic field whichreaches out beyond the immediate vicinity of these two filaments.

The structure which I have described above also eliminates variations inany leakage current which may occur between the sleeve II and the heaterwires I and 2. If any leakage current tends to occur from the sleeve IIto the wire I through the point of contact 20, a similar leakage currentwill occur between the wire 2 and the sleeve I I through the same pointof contact 20. Since with respect to any such point 20, the wires I and2 are substantially the same distance therefrom and are at equal andopposite potentials, the leakage current which occurred at each instantwill be substantially equal and opposite to each other. If there is anyresultant leakage current between the sleeve 20 and the wire pair I and2, this resultant current will be constant in value and will not followthe variations in voltage applied to the cathode heater. Of course it isto be understood that the point 20 may not always be exactly on thecenter line'between the wires I and 2. However, due to the fact that thedistance from the wires I and 2 to the outside of the insulating coating3 is preferably greater than the distance between these two wires, thepoint of contact between the insulating coating 3 and the interior ofthe sleeve I! can vary considerably on either side of the center point,as indicated at 20, and still have the wires I and 2 substantiallyequidistant from the point of contact. Slight diiierences in distancebetween the point of contact and the wires I and 2 will not introduceany serious resultant variation in the leakagecurrents, and thereforeany structure in which the contact between the insulating coating andthe cathode sleeve occurs at a limited area, which is substantiallyequidistant from two heating wires at substantially equal and oppositepotentials, comes within the purview of my invention. Due to thesymmetrical arrangement of the heater wires I and 2 with respect to theinterior of the cathode sleeve II in which the potentials on the variouswires are substantially equal and opposite to each other, the resultantemission current, if any, between the interior of the cath- .5 netic andelectrostatic fields around my novel heater, and have likewisesubstantially eliminated variations in leakage currents between theheater and the cathode sleeve. Such a cathode .when utilized in anelectrical space discharge device therefore introduces a minimum of huminto the operation thereof.

Of course it is to be understood that this invention-is not limited tothe particular details of the arrangement as described above as manyequivalents will suggest themselves to those skilled in the art. It isaccordingly desired that the appended claims be given a broadinterpretation commensurate with the scope of the invention within theart.

What is claimed is:

1. A cathode for an electrical space discharge device comprising acathode sleeve having an external electron emissive surface, a heaterinteriorly positioned within said sleeve, said heater comprising a pairof heater wires embedded side I by side in a unitary insulating coveringand connected together at one end thereof, said pair of wires-with itsinsulating covering being bent back'on itself to form two parallelclosely-adjacent heater sections, said pair of heater wires beingconnected to carry substantially equal heating currents therethrough inopposite directions.

2. A cathode for an electrical space discharge device comprising acathode sleeve having an external electron emissive surface, a heaterinteriorly positioned within said sleeve, said heater comprising a pairof heater wires embedded side by side in a unitary insulating coveringand connected together at one end thereof, said pair of wires with itsinsulating covering being bent back on itself to form a plurality ofparallel closely-adjacent heater sections, said pair of heater wiresbeing connected to carry substantially equal heating currentstherethrough in opposite directions.

3. A cathode for an electrical space discharge device comprising acathode sleeve having an external electron-emissive surface, a heaterinteriorly positioned within said sleeve, said heater comprising a pairof heater wires embedded side by side in a unitary insulating coveringand connected together at one end thereof, said pair of wires with itsinsulating covering being bent back on itself to form two parallelclosely-adiacent heater sections, the insulating covering of eachsection being adapted to contact with the interior of said sleeve alonga limited part of its cross-sectional perimeter at one side thereof, theregion of contact being substantially equidistant through saidinsulating covering to each of said heater wires, said pair of heaterwires being connected to carry substantially equal heating currentstherethrough in opposite directions.

4. The method of producing cathodes for electrical space dischargedevices which comprises coating individually with an insulating materialtwo wires, each of a length many times the length of wire to beincorporated into any one of said cathodes, placing said coated wiresside by side and coating them with an additional insulating coating,subdividing said coated pair of wires into a plurality of shorterelement pairs, each of which is of the length to beincorporated into oneof said cathodes, bending each of said element pairs into a plurality'ofparallel portions, and inserting it into a cathode sleeve.

5. The method of producing cathodes for electrical space dischargedevices which comprises coating individually with an insulating materialtwo wires, each of a lengthmany times the length of wire to beincorporated into any one of said cathodes, placing said coated wiresside by side and coating them with an additional insulating coating,subdividing said coated pair of wires into a plurality of shorterelement pairs, each of which is of the length to be incorporated intoone of said cathodes, connecting the two wires of each of said elementpairs together at one end thereof, and inserting said element pair intoa cathode sleeve.

6. The method of producing cathodes for electrical space dischargedevices which comprises coating individually with an insulating materialtwo wires, each of a length many times the length of wire to beincorporated into any one of said cathodes, placing said coated wiresside by side and coating them with an additional insulating coating,subdividing said coated'pair of wires into a plurality of shorterelement pairs,

each of which is of the length to be incorporated I into one of saidcathodes, connecting the two wires of each of said element pairstogether at one end thereof, bending each of said element pairs into aplurality of parallel portions, and inserting it into a cathode sleeve.

PAUL W. STUTSMAN.

